I know him best from The O.C., but there's also Lost, and Once Upon a Time, and Don't Be Afraid of the Dark, and Captain America: The Winter Soldier, and I've surely seen him in other things that I don't specifically remember.
I know him from Wings; and Eyes; and some voice work.
I mainly know him from Cheers, of course. But there's also 3 Men and a Baby (and its sequel); and Ink; and Mumford; and Becker; and The Good Place; and probably things I've forgotten.
I know him from The Cape; and season 6 of Community; and The Quick and the Dead; and Pitch Black; and an ep of Speechless; and some voice work. And surely I've seen or heard him in numerous other roles that I don't really remember.
I know from Return of the Jedi (Wicket); and Willow; and Prince Caspian & The Voyage of the Dawn Treader (Reepicheep); and The Silver Chair (Glimfeather); and The 10th Kingdom (Acorn); and Leprechaun; and an episode of Doctor Who (Porridge); and the Harry Potter movies (Filius Flitwick, et al.).
He was decent in Honey, I Shrunk the Kids: The TV Show, and a too-brief role in Heroes, and Terminator: The Sarah Connor Chronicles, and The Secret Circle.
de Lancie, John
I know him best as "Q" from Star Trek: The Next Generation, Deep Space Nine, and Voyager. Also there was The Hand That Rocks the Cradle; and an episode of Torchwood: Miracle Day; and some voice work.
I know him from The Muppet Movie; and Burke's Law; and a minor role in Blazing Saddles. But mainly I know him for his voice work.
De Niro, Robert
I know him from Taxi Driver; and Goodfellas (which I don't remember); and Mary Shelley's Frankenstein; and Hide and Seek; and Stardust; and Silver Linings Playbook; a small role in American Hustle; and Jackie Brown; and possibly some other stuff.
Mainly know him from Buffy the Vampire Slayer and Angel. But I've also seen him in a few eps of How I Met Your Mother; and Dollhouse; and Grimm. And he voiced Mirror Master in an ep of Justice League; and Dr. Quintum in All-Star Superman; and Nigel Taylor in Tarzan & Jane.
Enjoyed his work in The Flash, and later Becker, and on Boy Meets World for awhile. Oh, and of course he was also in the band "Hepcat," though I only know like one of their songs, I think.
I know him from Taxi; and Romancing the Stone; and Batman Returns; and L.A. Confidential; and Mars Attacks!; and Anything Else; and When in Rome; and a cameo in Austin Powers in Goldmember; and Big Fish; and voice work in Hercules; and Last Action Hero; and The Lorax; and surely lots of things I don't remember so well. Also a commercial for Nespresso.
Um, he was on Growing Pains for awhile. And there was Titanic; and The Beach; and Inception; and The Quick and the Dead; and The Aviator; and Romeo + Juliet; and Django Unchained; and Shutter Island. And I think he was in a Bubble Yum commercial I saw on one of my old VHS tapes and I was all, "OMG, that's Leonardo DiCaprio!" Or whatever.
I know him from Pitch Black; and The Fast and the Furious; and he voiced Groot in Guardians of the Galaxy; and the title character in The Iron Giant.
I know him from Elf; and Threshold; and Penelope; and Prince Caspian; and Knights of Badassdom; and Pixels.
Mainly think of him as the title character in Johnny Bago, which was pretty cool. Also Eddie in Lenny. I've probably seen him in some other stuff I don't really remember. I know I saw at least a bit of the series Cover Me. Shame I can't find any good links for him. He's someone who, if I'd seen more of his work, I might well consider a favorite...
I know him from Partners, and The O.C., and Nancy Drew, and an episode of Elementary, and he voiced the title character in Hercules (and its spin-offs). And I must have seen or heard him in other things I don't specifically remember him from.
Mainly know him as Worf from Star Trek: The Next Generation and Deep Space Nine, of course. And Star Trek VI, in which he played an ancestor of Worf's. And he's been in several episodes of Castle. Also various voice work.
Mainly think of him as Father in Beauty and the Beast. There was also an episode of Angel, and several eps of Picket Fences, and at least a couple eps of The Wizard, and The Cutting Edge, and probably various other stuff.
Downey Jr., Robert
I know him from the Iron Man movies, and Kiss Kiss Bang Bang, and Good Night, and Good Luck., and Sherlock Holmes, and A Scanner Darkly, and Weird Science, and Gothika; and The Soloist.
Mainly know him from The X-Files. There's also Aquarius, and Evolution. I think that's all I've seen.
I know him best from Evening Shade. But there's also The Muppet Movie; and Dark Night of the Scarecrow; and Dick Tracy; and O Brother Where Art Thou?; and several eps of Everybody Loves Raymond; and Mr. St. Nick; and When a Stranger Calls; and probably some other stuff I've forgotten. Also voice work as the narrator of Now & Again; and Francis Griffin in a few eps of Family Guy.